With the development of efficient light emitting diodes (LEDs) that emit blue light or ultraviolet or near ultraviolet light, commonly referred to herein as UV light, it has become feasible to produce LEDs that generate light through phosphor conversion of a portion of the primary emission of the LED (or the entire primary emission) to longer wavelengths. Conversion of primary emission of the LED to longer wavelengths is commonly referred to as down-conversion of the primary emission. In some systems, an unconverted portion of the primary emission of the LED combines with the converted light of longer wavelengths to produce the desired colored light, e.g., white light. Alternatively, the entire primary emission is converted to light with longer wavelengths and then combined to produce the desired light.
Conventionally, wavelength conversion of the primary emission of the LED is achieved using a phosphor that is held in a binding medium, such as epoxy, silicone, or other similar material. The phosphor is generally in the form of a powder that is mixed into the binding medium prior to curing. The uncured slurry containing the phosphor powder is deposited onto the LED to encapsulate the LED and subsequently cured.
It is desirable, however, to use high radiance phosphor-converted LEDs in many illumination applications, such as in projectors, automobile headlights, fiber optics and theater lights. One difficulty with phosphor encapsulated LEDs, however, is that the binding medium turns opaque and turns brown when subjected to high temperatures. This temperature limitation of the encapsulant, accordingly, limits the current at which the LED can be driven, which limits the radiance of the phosphor-converted LED.